Gamma ray detectors are used in a wide variety of apparatus, such as in positron emission tomograph (PET), single photon emission-computed tomograph (SPECT), explosive detectors, and the like. All of such apparatus depend upon, in part, detectors which can determine the position of interactions of gamma rays with the detectors, such that with a plurality of such position measurements, a scan of an object of interest can be made. These techniques are well known to those of skill in the art and will not, therefore, be further detailed herein.
The difficulty with all such detectors is that a large number of such detectors are necessary for accurate scanning, and the positions of interactions of gamma rays with the detectors must be determined with sufficient accuracy so that with the accumulation of, typically, millions of such determinations, sufficient data is obtained for producing an accurate image of the scanned object(s). Because each of the detectors must be capable of generating position data for a gamma ray interaction, acquisition of such position data and the compilation thereof, e.g. by a computer, may require very substantial and expensive apparatus.
Typically, the data from such detectors is initiated by a generation of light in a scintillator material produced within the detector as a gamma ray interacts in the detector. By determining the detector in which such light was emitted, and the position of that emitted light within the detector, a data point for a scan is produced. By providing a multiplicity of such detectors, which can number in the thousands depending upon the application, and each detector providing a multiplicity of data points, the compiled interaction data can be used to produce an image of the scanned object. In PET, two such detectors which fire simultaneously are used to create a line of response (LOR) used in constructing the image.
Typically, for example, photodetectors will be provided with an array of scintillating detectors where the photodetectors detect the emission of light in the scintillating detectors with a logic circuit being employed to determine the position of emitted light. However, as can be appreciated, the monitoring instrumentation, including the photodetectors, logic circuits and related controller and signal devices (referred to collectively as a read-out channel) can result in very complex monitoring instrumentation, especially when a large number of detectors are required to obtain the desired accuracy in the intended scan.
In addition, a conventional detector for such gamma ray scanning devices is an inorganic scintillating crystalline material, e.g. cerium doped lutetium oxyorthosilicate (LSO) and bismuth germinate (BGO), which is, in and of itself, expensive. The stimulated region of the scintillator material which will emit light and thereby allow the position of interaction with a gamma ray to be determined. The X-Y position resolution of such conventional detectors may be on the order of 20 mm2 and doe not tend to be uniform for all positions.
This resolution and lack of uniformity results in a basic level of inaccuracy that precludes precise identification of exactly where in the detector, i.e. at which XY coordinates, that gamma ray interaction occurred. In addition, the depth of the interaction, i.e. the Z coordinate, is generally not determined, or is determined with reduced accuracy in comparison to the X and Y coordinates, resulting in a so-called parallax error and further reducing the accuracy of the resulting image.
A modular light signal triggerable gamma ray detector is disclosed in the Applicant's U.S. Pat. No. 6,100,532, which is hereby incorporated by reference in its entirety. The disclosed detector includes at least one module, and each module includes a converter for converting gamma rays into charged particles. A scintillator is provided for emitting light in response to the charged particles produced by the converter and an associated photodetector determines when light has been emitted from the scintillator. A two-coordinate position detector is provided for determining the X, Y and Z coordinates of charged particles interacting with the position detector. Finally, a controller and signal device are provided for signaling the presence of emitted light in the photodetectors and for activating the position detector to complete a system that addresses some of the deficiencies of conventional systems. The resulting gamma ray detector is generally less expensive to construct, reduces the amount of monitoring instrumentation necessary for acquiring the required data, and more accurately determines the X, Y and Z coordinates of the gamma ray interaction than conventional systems.
The conversion of gamma rays in material (including heavy liquids like xenon (Xe), krypton (Kr), and the like) and the production of scintillation light and charged products (electrons and positrons) are well studied and understood by those skilled in the art and will not, therefore, be discussed in detail herein. It is also noted that various software tools are available for simulating the interactions of gamma rays and charged particles with a range of matter. Position sensitive detectors for charged particles, such as noble liquid ionization chambers, time-projection-chambers (TPC), and light detection arrays are conventional instruments that are known to have position and energy resolution capability similar to the preferred detectors and may be suitable for use, singly or in combination, in the present apparatus and method.
Improved liquid Xe position sensitive ionization detectors with grids such as described by Masuda et al. in A Liquid Xenon Position Sensitive Gamma-Ray Detector for Positron Annihilation Experiments, Nucl. Instr. Meth. 188 (1981) 629-638; and Test of a Dual-Type Gridded Ionization Chamber Using Liquid Zenon, Nucl. Instr. Meth. 174 (1980) 439-446, the contents of which are hereby incorporated, in their entirely, by reference, can provide sub-millimeter position resolution for low energy gamma rays. Such detectors have been incorporated in gated time projection ionization chambers as reported by Columbia University that have achieved position resolution on the order of 1 mm and energy resolution on the order of 5.9% for 1 MeV gamma ray energy.
Others have constructed a liquid Xe ionization detector having a transaxial position resolution on the order of 1 mm, depth of interaction resolution of 5 mm, coincidence time resolution of 1.3 ns, energy resolution at 511 keV of 17% and efficiency of approximately 60% as reported be Lopes, et al., in Positron Emission Tomography Instrumentation: Development of a Detector Based on Liquid Xenon, Proc. Calorimetry in High Energy Physics, pages 675-680 (1999)), the contents of which is hereby incorporated, in its entirety, by reference.
Positron Emission Tomography (PET) is an important medical imaging modality in which pairs of gamma rays emitted when positrons annihilate are detected in coincidence. Data obtained from coincident detectors recorded within a time window characteristic of the specific apparatus employed are used to construct lines of response (LOR) from which images are developed using well known algorithms. Images are generally limited in resolution by several factors including range of positrons, detector spatial and energy resolutions, scattering of photons in the object/patient under investigation before the photons reach the detectors, and scattering in the detectors. In addition, random or accidental coincidences which occur when two or more photons from separate annihilation events are detected within the resolving time window of the apparatus limit the statistical precision of image reconstruction. Identification and/or suppression of data associated with random or accidental coincidences will, therefore, tend to improve the accuracy and image quality that can be obtained from a given detector system.